That's only true in DC power. Since AC power drain includes impedance (and, obviously, the power is running in a waveform), the waveforms of the volts and amps are not aligned. This leads to variable wattages in an AC power line, even some that are negative, conceivably. In fact, the mythical perfect inductor would actually draw no watts, alternatingly drawing and pushing back the same amount continuously.

I'm probably doing a poor job of explaining it, so check out this page on power factor, which is inextricably linked to what I'm trying to talk about here, as power factor is the ratio of Watts to VoltAmps. He does a good job of explaining it, I think.

Edit: Then again, motherboards and other inside-the-case computer devices should be drawing DC power, so what do I know? Maybe the Watts are measured at the mains-side and other stuff is measured elsewhere. Regardless, that power factor stuff is interesting to know, so I haven't totally wasted my time.